Large-eddy simulation of flow over rough channel beds

摘要

In this paper we present the results of several large eddy simulations (LES) of the flow over rough channel beds. The wall roughness is introduced via artificial 2-dimensional square bars, positioned along the channel at varying spacing. The spacing is chosen such that the three roughness types, isolated roughness, wake interference roughness, and quasi-smooth-wall roughness are represented. The vertical velocity profiles exhibit the well known feature of turbulent flows over rough wall flows i.e. a shift of the logprofile above the roughness crests. However, the increment of the shift depends strongly on the arrangement of the artificial roughness elements. In addition, turbulent fluctuations and Reynolds stresses show a dependency on the type of the roughness. Similar to previous experimental and numerical investigations the occurrence of flow into and out of the cavities is shown and vortices are shed into the outer flow. These vortices are associated with the occurrence of low and high speed streaks, a feature that is also found in smooth wall flows. However, the shape and length of these streaks depend strongly on the roughness type.